This invention relates to a system and process or method for maintaining the position of a floating offshore installation with respect to a seabed site. More specifically, this invention relates to a system and method for geometrically determining the position of a floating offshore installation over a wellhead.
In the past, offshore installations have been extensively utilized around and upon the continental shelf regions of the world. Examples of offshore facilities include supports for radar stations, light beacons, scientific and exploration laboratories, chemical plants, power generating plants, mining stations, etc. Principally, however, offshore facilities have been utilized by the oil and gas industry in connection with drilling, production and/or distribution operations.
In the initial stages of offshore drilling/production activity, operations were conducted along the near shore portions of the Gulf of Mexico in swamp or marshlands and seaward to water depths of 100 feet or more. In such depths, fixed length towers or platforms have been extensively utilized.
In an attempt to keep pace with a burgeoning worldwide energy demand, more recent offshore activity has become decidedly more aggressive. In this regard, floating drillships which are dynamically positioned and/or turret moored have operated in water depths of up to approximately 5,000 feet. If the present trend of spiralling price levels for crude oil continues, the only offshore depth limitations for drilling and production of oil will be the technological capability of the equipment.
Drillship operations are conducted vertically through a moon pool, or a well, in the ship hull. A riser is lowered through the moon pool from one or more tensioner rings to the seabed and a bit and drill string are lowered concentrically within the riser for drilling into the seabed. During drilling, a compressed, mud slurry, is continuously pumped down the drill string, out the bit and back up the riser in the annulus between the exterior of the drill string and interior of the riser. In deep water, the weight of the riser column can become substantial. Accordingly, it is critical to maintain the riser in a generally vertical posture to prevent over stressing and "kinking" the riser conduit.
In the past, various systems and methods have been envisioned for determining the position of a vessel relative to a seabed site. However, as will be seen below, such systems and methods have been burdened by various disadvantages, including inaccuracy, lack of economy, and inefficiency or ineffectiveness.
One previously known system is directed to horizontally stabilizing a floating structure with strain gauge sensors secured to a casing at a point above and adjacent to the seabed. These gauges are connected to electric motors on a platform located at the water's surface. Sensed movement of the casing activates assemblies for stabilizing the horizontal position of the drilling platform. Such a system has several disadvantages, primary of which resides in the fact that the system is not designed so as to provide accurate positioning information for determining the position of the platform.
Other systems disclose vessel positioning by the use of an assemblage which is movable along a drill string so as to determine the angular deviation from the vertical at various elevations along the drill string. In this regard, the angular deviation from the vertical is sensed by a pendulum apparatus which generates electrical signals indicative of position. These electrical signals are then utilized to determine the relative angular displacement of the pendulum with respect to an assemblage carriage. However, such a system is also burdened with disadvantages. First, this system is hampered, in its effectiveness, by lack of accuracy in computing the position information. Second, the system is, by its very nature (being mechanical in part), subject to error induced by outside elements (for example, rough sea, bad weather, etc.).
Still further acoustic systems have been utilized for dynamically positioning vessels. One system comprises a ship, used as a drilling platform, which is dynamically positioned through the use of acoustic transmitters mounted on the base of a well. Signals produced by the transmitters are received aboard ship by one or more microphones. Whereas such acoustic systems represent an advance over previously known devices, such systems can conceivably fail to function (that is, experience a "blackout") during which failure time data cannot be obtained. Moreover, such previously known systems are still somewhat burdened by inaccuracy in the process of position determination.
In yet another acoustic system, a drilling platform is positioned over a bore hole and a plurality of acoustic transponders are positioned on the seabed around the bore hole. Corresponding transducers are mounted on a lower end of a spar buoy hull for receiving signals. This system also features the use of a photoelectric position indicating device, which can be used to position the buoy. Again, such an acoustic system possesses certain disadvantages such as previously noted.
Recent emphasis on drilling operations in increasingly deep waters has resulted in increasing concern with prior systems. In this regard, it would be desirable to provide positioning systems and methods which have superior reliability. Additionally it would be highly desirable to provide a reliable position reference to be used as a backup for acoustic reference system. Various techniques, such as a taut wire, lower riser angle, inertial reference and radio systems, have been used as back-up systems with varying success, depending upon the water depth and the weather conditions. In many cases, however, the high cost and/or low reliability of such back-up systems have effectively precluded them from serious consideration.
The difficulties suggested in the preceeding are not intended to be exhaustive, but rather are among many which tend to reduce the effectiveness and user satisfaction of prior positioning systems. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that positioning systems and processes, particularly of the mechanical, electro-mechanical and acoustic type, appearing in the past will admit to worthwhile improvement.